Built Right Detail 05: Foundation-to-Wall Transition

A builder’s net zero envelope course by Xerox Builders · October 2026

The base of the wall is where the wood frame meets the concrete foundation, and it is buried quickly by backfill and cladding. This Detail shows how to break the capillary path under the sill plate, seal the air at the sill inside and out, lap the water layers over a base flashing, and keep cladding clear of the ground.

Four jobs at the base of the wall

The base of the wall is where the wood frame sits on the concrete foundation. It has to do four things at once: seal the air, stop moisture wicking up from the concrete, shed rainwater out of the wall, and keep the wood and cladding clear of the ground. It also gets buried quickly, first by backfill and then by cladding, so it has to be right the first time.

1. Break the capillary path

Concrete is porous. It draws moisture up from the soil, and wood sitting directly on it stays damp. Part 9 of the BC Building Code requires untreated wood resting on concrete that is in contact with the ground to be separated from the concrete by polyethylene film or Type S roll roofing, unless the wood is at least 150 mm above the ground. Many builders use a full-width foam sill gasket under the sill plate. It seals air and acts as a break, but check with your building official that the product you use is accepted as the separation layer, or add poly or roll roofing as well.

2. Seal the air, inside and out

Building Science Corporation and others identify the concrete-to-sill connection as one of the most common sources of air leakage in houses. Part 9 requires the joint between the sill plate of an exterior wall and the foundation to be sealed. Three joints matter:

1. Concrete to sill plate
2. Sill plate to rim joist
3. Rim joist to subfloor

Use a gasket, sealant or both. Concrete is rarely flat, so on rough or uneven foundation tops run beads of sealant under the gasket to fill the low spots.

If your primary air barrier is on the outside (see Detail 01), it must connect to the concrete. A strip of self-adhered membrane bridges the sheathing, the sill joint and the face of the foundation. The concrete must be clean, dry and primed, and the membrane, primer and any sealant must be compatible with each other (Detail 02). Setting the sheathing flush with the face of the foundation gives a flat surface that is much easier to tape. The wall air barrier and the water-resistive barrier (WRB) then lap over the tie-in.

3. Shed the water: lap the layers

The base of the wall follows the same shingle rule as a window opening (Detail 07): every upper layer laps over the layer below.

Part 9 requires flashings to extend at least 50 mm upward behind the sheathing membrane, slope to the outside and finish with a drip. The code allows an insect screen across the bottom of the rainscreen cavity as long as it still lets the cavity drain and vent.

4. Keep it clear of the ground

Part 9 requires at least 200 mm between finished ground and cladding that is damaged by moisture, such as untreated wood, plywood, OSB and hardboard. Slope the ground away from the house. Be careful with work that happens after framing: landscaping, paving, planters and patios often raise the grade near the wall. Check the clearance at final grade, not just rough grade, and send downspouts to the drainage system rather than onto the base of the wall.

With exterior insulation

• Line up the foundation insulation with the wall insulation so there is no cold gap at the sill and rim.
• Foundation insulation above grade needs a hard, durable finish to protect it from impact and sunlight.
• The base flashing must reach out past the insulation and the cladding, with a drip, so water clears everything below it.
• Use pressure-treated or naturally durable strapping near grade, and confirm fastening through the insulation with your designer.

Codes change and cities add their own rules. Confirm the current requirements with your building department, designer or envelope consultant.

Site checklist

• Gasket or capillary break under every sill plate
• Concrete-to-sill, sill-to-rim and rim-to-subfloor joints sealed
• Sealant beads under the gasket where the concrete is rough
• Membrane tie-in fully bonded to clean, primed concrete
• Base flashing sloped out, with a drip, upstand behind the WRB
• WRB lapped over the base flashing
• Insect screen in place and cavity open to drain
• At least 200 mm cladding-to-grade clearance at final grade
• Base of wall photographed before backfill and cladding

Test yourself

Try to answer each question, then click it to check.

Next: Detail 06, Rim Joists & Floor Lines. See also our Basement & Foundation Waterproofing Field Guide.